In solid oxide fuel cell (SOFC) stacks, especially planar stacks with metallic interconnects, contact resistance between the electrodes, especially the cathode, and the metallic interconnect is a major factor in stack performance losses and long-term performance degradation. Conventionally, ceramic contact layers with compositions similar to cathode materials have been used to minimize contact resistance. Unlike the cathode, which has been sintered at high temperatures (950° C. to 1200° C.), it is not ideal for ceramic contact layers to be exposed to high temperatures after stack assembling, otherwise the metallic interconnects will become severely oxidized. When exposed to typical fuel cell operating temperatures (650° C. to 800° C.) the ceramic contact layers exhibit low conductivity and poor adhesion to both the cathode and interconnect. It has been reported in literature that contact layers can contribute up to 40 to 50% of total performance loss in a planar SOFC stack.
In conventional SOFC stack designs, all the cells are connected in series to achieve useful electrical voltage and power. The serial connection of SOFCs has, especially at high power densities, principle related drawbacks. For example, the power output of the whole stack can collapse if a breakdown of one single cell, interconnect, or seal occurs. Due to the configuration it is not always possible to shut down, swap out, or bypass the defective or failed cells during the stack operation. Additionally, when connected in series, all the cells operate at the same current load but different voltages, depending on the internal resistance of each cell. SOFCs with high internal resistances operate at lower voltages. It is well known that SOFC degradation is strongly affected by the operating voltage. Low operating voltage results in a distinct increase of degradation rate. As a result, low performing cells experience higher degradation rates and fail faster than high performing cells.
Additionally, achieving and maintaining a gas tight seal at high temperatures is extremely difficult. Most stack failures can be directly related to sealing issues. Glass is conventionally used as the sealing material. However, glass is rigid, brittle and can easily fail during thermal cycling. In addition, the additives of glass (Al, Si, B, etc. . . . ) can readily migrate and react with cell materials in SOFCs, producing undesired phases and increasing resistance. Compressive seals (i.e. mica-based gaskets) exhibit higher leak rates than glass seals and typically require high pressure.
There exists a need to design a novel SOFC stack design that eliminates or reduces the issues concerning conventional SOFC stack designs.